The present invention relates to methods and systems for placing sensors beneath the earth""s surface to allow monitoring of subsurface properties. In particular, the invention relates to methods and systems for monitoring the movement of fluids in reservoirs, such as hydrocarbon reservoirs.
In certain situations, it is desirable to provide sensors for long term or permanent monitoring of subsurface formations. Examples include environmental monitoring, water flow monitoring, seismic monitoring and hydrocarbon reservoir management. In the latter case, the information obtained from permanent or long term monitoring is used to manage the production from the wells in a given region in order to optimise oil or gas recovery. A review of permanent monitoring applications is given in the article Permanent Monitoring-Looking at Lifetime Reservoir Dynamics published in Oilfield Review, Winter 1995 pp. 32-46.
There have been certain proposals for installation of permanent sensors in oil or gas wells or for the monitoring of hydrocarbon reservoirs. One example is found in U.S. Pat. No. 5,662,165 which describes a downhole control system for a production well which is associated with permanent downhole formation evaluation sensors such as neutron generator, gamma ray detector and resistivity sensors. The data retrieved from the sensors can be used to determine corrective action to be taken to maintain effective production from the well. Since the sensors are placed in the producing well, the depth of investigation into the formation is limited by the depth of investigation of a given sensor. Thus effective measurement of far field properties is prevented. The disadvantage in this approach is that it in only possible to react to a change experienced very close to a given well, not to anticipate the change and take preventative action. Other disadvantages are that the presence of casing can interfere with a measurement. It has been proposed to install sensors behind casing but these are susceptible to damage during perforating and still are incapable of making far field measurements. WO 98/50680 and WO 98/50681 describe the use of fibre-optic based sensors in permanent installations to monitor formations surrounding producing wells. While the sensors are relatively cheap and long-lived, they still suffer from the inability to see into the far-field of the well.
In certain reservoirs, it is necessary to attempt to provide some means for driving the in situ hydrocarbons into the producing well. This is known as xe2x80x9csecondary recoveryxe2x80x9d and two common examples of this are water flooding and steam flooding. In such cases, water or steam are injected into the formation through one or more injection wells placed some distance from the producing well(s) and move through the formation to the producing wells, driving the oil in front of it. In the case of steam, the heat provided also improves the mobility of the oil in the formation. On problem with such methods is that often the flood front reaches the production well bypassing oil in the formation (this is sometimes known as xe2x80x9cbreakthroughxe2x80x9d). In order to control the process to avoid breakthrough it is desirable to monitor the progress of the flood front. However, monitoring from the production well as described above does not see far enough into the formation to allow remedial action to be taken to prevent breakthrough.
In steam flood secondary recovery, one measurement which has been made is that of temperature near the producing well(s) to determine the approach of the steam front. Other measurements which might be useful are: pressure, mechanical and electrical properties of the formation. FIG. 1 shows one system for measuring temperature in which a U-shaped 0.25xe2x80x3 stainless steel tube 10 is run along the outside of the production well casing 12 where it is cemented in place with the casing in the hole 14. A fibre optic sensor 16 is then installed by pumping nitrogen through the U-tube 10 until the fibre 16 is in place, at which time temperature measurements can be made by connecting the ends of the fibre 16 to a source and receiver instrument 18 at the surface. The potential for damage to the U-tube is high, either in the installation process, or during perforating and again, only near-field measurements can be made.
One approach to avoiding the problem of making far field measurements is found in WO 98/15850 which proposes the drilling of non-producing boreholes for positioning permanent seismic monitoring sensors. The trajectories of the boreholes are chosen to optimise the response of the sensors to seismic signals rather than production from the reservoir. Seismic measurements should be able to monitor the flood front, particularly a steam flood front. However, the requirement to drill horizontal boreholes makes the drilling of these boreholes a relatively complex and expensive proposition. In order to accommodate seismic sensors, it is necessary for the borehole to have a sufficiently large size in view of the size and physical requirements of the systems used. Furthermore, making seismic measurements is relatively expensive and time consuming and is not applicable to a permanent monitoring solution.
Most boreholes are constructed using the well-known rotary drilling technique common in the oil and gas industry. One alternative when drilling smaller diameter holes is to use a technique called coiled-tubing drilling in which a drilling bottom hole assembly (BHA) is connected to the end of a continuous tubing through which a fluid is pumped to drive a downhole motor in the BHA to turn the drill bit. The basic technique is reviewed in the article entitled An Early Look at Coiled-Tubing Drilling published in Oilfield Review, July 1992, pp. 45-51. While the technique has been applied mainly to re-entry drilling, new exploration wells have been drilling using this approach. Coiled tubing has also been used to convey logging instruments into boreholes and to place fluids or equipment at precise locations in boreholes. One approach to long term monitoring is found in U.S. Pat. No. 5,860,483 which describes the use of coiled tubing to drill holes for locating seismic sensors. The seismic sensors are mounted on the outside of the coiled tubing. After drilling, the coiled tubing is withdrawn from the hole and the drilling tools removed. It is then reinserted into the hole, which can be allowed to collapse around it.
The present invention attempts to provide a solution to far-field monitoring of formations surrounding producing boreholes, especially in cases where enhanced recovery techniques are used.
The present invention resides in the use of coiled tubing to drill into the formation and provide a conduit back to the surface to allow sensors to be deployed and measurements made for monitoring of the formation.
One aspect of the invention provides a method of monitoring subsurface formation properties between injection and production wells. In this method, coiled-tubing is used to drill sensor holes at predetermined positions between the injection and production wells and the coiled-tubing is permanently fixed in the hole such that a sensor can be deployed in the tubing to provide measurements of the formation.
Various options are available within the scope of this method. Typically, a bottom hole assembly incorporating drilling tools will be attached to the coiled tubing for use in drilling the hole. When the hole has been drilled to depth, the coiled tubing can be withdrawn, the BHA removed and the tubing reinserted into the hole where it is cemented in place. Alternatively, a different coiled tubing can be installed in the hole. Also, the BHA can be left in the hole so that it is not necessary to withdraw the tubing from the hole before completion. The particular option chosen will depend on matters such as cost, convenience, nature of sensors used, etc.
For monitoring progress of steam flood, it is convenient to use a continuous fibre optic sensor which measures temperature. The particularly preferred option is a fibre optic sensor which runs from the surface, down the length of the coiled-tubing and back to the surface (i.e. and elongated xe2x80x9cUxe2x80x9d shape). Such sensors can either be permanently installed in the coiled-tubing or can be deployed on a temporary basis in each coiled tubing in turn. In the former case, the sensor can be located in the coiled-tubing used to drill the hole, whether the BHA is left in situ or removed. In the latter case, the fibre optic sensor can be attached to a plug which is pumped down the coiled tubing. After the measurement has been made, the plug can be detached and the fibre optic sensor retrieved and used again in another well. In another embodiment, sensor tubes are run into the coiled tubing and the sensors pumped along these so as to be positioned in the formation when required. A single sensor tube or a double, U-shaped tube can be used as appropriate.
Another aspect of the invention provides a method of monitoring a steam flood operation comprising positioning a number of sensor holes between one or more injection wells and one or more producing wells using a method as described above and measuring the temperature of the subsurface formation either continuously or from time to time using a fibre optic sensor deployed in each hole.